1. Field of the Invention
This invention relates generally to circuits designed to digitize a parameter having an unknown value.
2. Description of the Related Art
There are many applications in which it is desirable to digitize the value of a parameter having an unknown value. For example, a resistor may be used as a means to configure an integrated circuit (IC) to operate in one of several possible modes of operation determined by the value of the resistance. When the configuration means requires that the resistance value be presented in digital form, a resistance digitizer (or resistance-to-digital (R-to-D) converter) is needed.
One possible implementation of an R-to-D converter uses a dedicated ‘reference resistor’, in which a circuit compares the resistance of the resistor to be measured to that of the resistor that is used as a reference. This requires a reference resistor of known value; any error between the actual and assumed values of the reference resistor will translate into a digitization error. Fabricating accurate resistors to be used as reference resistors in an IC is difficult because the accuracy to which they can be (cheaply) fabricated is around ±20%. Additionally, such resistors typically have a significant temperature dependency. Therefore, it is difficult to achieve a digitization accuracy better than about ±20% (plus additional errors across temperature) when digitizing the value of a resistor using a reference resistor fabricated on an IC.
Alternatively, a discrete reference resistor of high accuracy and low temperature coefficient could be used, external to the IC. However, a disadvantage of this approach is that it requires that an I/O pin on the IC be available for connection to the external reference resistor (in addition to the pin necessary to connect the resistor to be digitized).
If a dedicated reference resistor is to be avoided, two possible approaches to digitizing a resistance are to force a current into the resistor and digitize the resulting voltage, and to force a voltage across the resistor and digitize the resulting current. The application of either of these two approaches can be difficult in the presence of a capacitance of arbitrary value shunting the resistance to be digitized. One reason is that forcing a current into the resistor is speed-limited by the RC product of the resistance and the shunt capacitance. For example, the digitization of a 200KΩ resistor with 40 nF of shunt capacitance requires almost 37 ms of settling time before the settling error is below 1%. Forcing a voltage across the resistance to be digitized enables much faster operation, assuming that the Thévenin equivalent of the subcircuit used to force the voltage has a small output impedance. However, forcing a voltage across the resistance is best done using a feedback loop, the stability-compensation of which will have to contend with the difficulties of the pole introduced by a shunt capacitance of arbitrary value.
Note that, though an R-to-D converter has been discussed, problems of a similar nature can affect converters designed to digitize other parameters of unknown value, such as a voltage or current.